What does it mean " yankin' my chain"
Amused, the trucker cocks his head and says, "You yankin' my chain, young fella?"

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yanking = pulling

To pull someone's chain = to try and trick someone

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"are you yanking my chain" = "are you kidding me?

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It's a response to a statement you believe is untrue. More specifically, one that was told to get an emotional reaction from you.

We can't go to the concert because I lost our tickets.
If you wash the dishes, I'll give you a thousand dollars.